1. Field of the Invention
The present invention relates to a portable battery-driven projector (projection imaging equipment). In particular, it relates to a lighting circuit for a high-pressure discharge lamp used as a light source.
2. Description of the Related Art
A direct-current high-pressure discharge lamp (high-pressure mercury vapor lamp) used as a light source of a projector generally has a high rated power (120 W or more, for example) and is considered difficult to drive by a battery. Thus, many projectors are connected to the commercial alternating-current power supply (AC 100 V, for example) and convert the alternating-current power to the direct-current power by an AC/DC converter for lighting.
FIG. 3 is a block diagram showing a conventional lighting circuit for a high-pressure discharge lamp. As shown in this drawing, the lighting circuit comprises an AC/DC converter 50, a ballast circuit 51, a power control section 52, alighting initiation circuit 53, and a high-pressure discharge lamp 54. Light emitted by the high-pressure discharge lamp 54 generates an image through an optical engine 55, and the image is projected onto a screen S.
Because the sealing technique for the high-pressure discharge lamp has been improved in recent years, higher mercury vapor pressure can be achieved during stable lighting. Thus, the light emission efficiency has been improved. Consequently, high-pressure discharge lamps having a rated power of 100 W or lower have been developed. However, the rated power of 100 W is still considered too high to use a battery to drive the lamp.
In the Documents of the related art 1, 2 and 4 described below, battery-driven projectors are disclosed, and in the Document of the related art 3, a discharge lighting device for a battery-driven liquid crystal backlight is disclosed. According to the techniques proposed in these documents, all the electric power excluding the part for the light source is supplied from the personal computer to reduce power consumption, or the brightness of the light source is decreased in response to the decrease of the remaining battery power.
[Document 1]: Japanese Patent Laid-Open No. 2004-69997
[Document 2]: Japanese Patent Laid-Open No. 2003-149733
[Document 3]: Japanese Patent Laid-Open No. 8-330081
[Document 4]: Japanese Patent Laid-Open No. 8-47261